Gangliosides, sialylated glycosphingolipids found on all vertebrate cells and tissues, play well-established roles in diverse molecular signaling pathways that impact human diseases including diabetes, cancer, neuro- degenerative proteinopathies, intellectual disability, and many others. The major ganglioside structures are well-defined, finite, and shared across vertebrate species. Their glycans regulate cell signaling independent of a protein carrier. Most gangliosides reside on the cell surface with their ceramide lipids embedded in the plasma membrane and their glycans extending outward. They regulate cell physiology in two modes, cis and trans. As cis regulators, they associate laterally via glycan binding to transmembrane proteins in the same cell membrane to regulate their function. As trans recognition molecules they engage proteins in the extracellular milieu or on apposing cells, mediating cell-cell interactions. Both cis and trans interactions are specific for ganglioside glycan structures and essential for human health. Most ganglioside functions and ganglioside-protein interactions remain poorly understood due to lack of broadly accessible, adaptable and validated tools and optimized methods for their use. It is the goal of this project to address this need using chemical biology technologies to synthesize a defined set of major gangliosides carrying minimally disruptive bifunctional photoreactive and alkyne (click chemistry) tags. Optimized and validated protocols will be generated to deliver bifunctionally tagged gangliosides to the outer leaflet of the plasma membrane of cells, where most gangliosides reside and function. Our goal is to synthesize the ganglioside probe toolkit, validate appropriate cell delivery of the probes, validate their use to identify glycan-specific ganglioside binding proteins, and transfer the reagents and protocols to other biomedical research laboratories within the term of this project. The deliverables, ganglioside probes and validated methods for their use, will be distributed broadly to biomedical researchers to discover the identities, specificities, distributions and functions of ganglioside binding proteins relevant to a variety of human cells, tissues, and diseases.